Insect Conservation and Diversity (2016) 9, 456–469 doi: 10.1111/icad.12183 Complementary distribution patterns of arthropod detritivores (woodlice and millipedes) along forest edge-to-interior gradients PALLIETER DE SMEDT,1 KAREN WUYTS,2 LANDER BAETEN,1 AN DE SCHRIJVER,1 WILLEM PROESMANS,1 PIETER DE FRENNE,1 EVY AMPOORTER,1 ELYN REMY,1 MERLIJN GIJBELS,1 MARTIN HERMY,3 4 1 DRIES BONTE andKRISVERHEYEN 1Forest & Nature Lab, Department of Forest and Water management, Ghent University, Melle (Gontrode), Belgium, 2ENdEMIC Research Group, Department of Bioscience Engineering, University of Antwerp, Antwerp, Belgium, 3Division Forest, Nature and Landscape, Department of Earth and Environmental Sciences, University of Leuven, Leuven, Belgium and 4Terrestrial Ecology Unit (TEREC), Department of Biology, Ghent University, Ghent, Belgium Abstract. 1. Worldwide, forest fragmentation induces edge effects, thereby strongly altering the forest microclimate and abiotic characteristics in the forest edge compared to the forest interior. The impact of edge-to-interior gradients on abiotic parameters has been extensively studied, but we lack insights on how biodiversity, and soil communities in particular, are structured along these gra- dients. 2. Woodlice (Isopoda) and millipedes (Diplopoda) are dominant macro-detriti- vores in temperate forests with acidic sandy soils. 3. We investigated the distribution of these macro-detritivores along forest edge-to-interior gradients in six different forest stands with sandy soils in north- ern Belgium. 4. Woodlouse abundance decreased exponentially with distance from the for- est edge, whereas millipede abundance did not begin to decrease until 7 m inside the forest stands. Overall, these patterns were highly species specific and could be linked to the species’ desiccation tolerance. Whereas the observed abundance patterns were independent from forest stand and dominant tree species, tree species had a large effect on community structure. 5. Edge gradients in macro-detritivores may consequently have implications for nutrient cycling, especially in smaller forest fragments with a large edge-to- interior ratio. Key words. Detritivores, diplopoda, edge effects, gradients, isopoda, soil fauna. Introduction (Janzen, 1986; Reed et al., 1996; Gascon et al., 2000; Harper et al., 2005; Fletcher et al., 2007; Echeverria et al., Forest habitats are profoundly fragmented around the 2008). Forest edges are characterised by enhanced light world (Wade et al., 2003). Such fragmentation induces a availability (Delgado et al., 2007), higher wind speeds reduction in forest patch sizes and strengthens edge effects (Wuyts et al., 2008a), higher air and soil temperatures (Delgado et al., 2007; Heithecker & Halpern, 2007), and lower relative humidity and soil moisture (Chen et al., Correspondence: Pallieter De Smedt, Forest & Nature Lab, 1995; Gehlhausen et al., 2000). They are also relative ‘hot- Department of Forest and Water management, Ghent University, spots’ for the deposition of eutrophying and acidifying Geraardsbergsesteenweg 267, 9090 Melle (Gontrode), Belgium. atmospheric pollutants (Weathers et al., 2001; Wuyts E-mail: [email protected] et al., 2008b,c) compared to the forest interior. The 456 Ó 2016 The Royal Entomological Society Arthropod detritivores in forest edges 457 magnitude and depth of influence of these edge effects are edge effects and macro-detritivore communities, we put strongly affected by the structure and composition of the forward the following hypotheses about the influence of edge itself (Weathers et al., 2001; Wuyts et al., 2008b,c). edge effects on woodlouse and millipede distribution: These abiotic edge gradients then give rise to secondary 1 More favourable environmental conditions (higher effects on biotic effects at the edge of forest ecosystems temperatures and higher litter quality) at the forest (Murcia, 1995; Harper et al., 2005). edge will result in a higher abundance of macro-detri- Soil macro-invertebrates are dominant detritivores in tivores at this boundary, with abundance steadily temperate forests, which breakdown dead organic material declining towards interiors. If true, we expect the mass (Lavelle, 1997), thus affecting the physico-chemical char- of the ectorganic horizon to show the inverse trend acteristics of soil (Snyder & Hendrix, 2008). By reducing due to increasing rates of decomposition from the inte- the size of dead organic material on the forest floor rior to the forest edge. (Anderson, 1988; Grelle et al., 2000), they increase the 2 The response of macro-detritivores to edge proximity accessible surface area for further decomposition by is species specific, as each species exhibits different microbes (Harper et al., 2005). This results in a more temperature and humidity preferences. stable soil organic matter layer (Wolters, 2000). The trans- 3 Responses of detritivores to forest edge proximity can formation of fallen leaves into macro-detritivore faeces be related to changes in relevant abiotic parameters also has strong effects on the microbial response and con- (i.e. food quality, cation content of the soil, etc.). sequently on the breakdown of the leaf material (Joly et al., 2015). Exclusion of these soil macro-arthropods slows down decomposition rates (Riutta et al., 2012; Slade & Riutta, 2012), and their presence is therefore of vital Material and methods importance for nutrient cycling in forest ecosystems. The distribution of macro-arthropods, such as woodlice and Site description millipedes, within forests is highly scattered (Hornung, 2011) and aggregated towards forest edges (Riutta et al., We selected six forest stands in the northern part of 2012). The occurrence of abiotic edge effects raises the Belgium, located on poor, acidic, well-drained sandy soils question of whether the response from macro-arthropods (Haplic podzols) of the Campine and Sandy region. All also varies gradually as the forest edge is approached, and were recently created forests, formerly managed as heath- whether this is reflected in the accumulation of forest floor lands until 80–90 years ago. This heathland management material. Yet our understanding of how biotic factors, practice resulted in a significant depletion of soil nutrients such as different taxonomic groups of the soil fauna com- through sheep grazing and turf cutting on these already munity, change along edge-to-interior gradients and how naturally nutrient-poor soils. After some years of aban- this could affect the litter decomposition process is extre- donment, the sites were afforested with monocultures of mely limited (Hattenschwiler€ et al., 2005). Temperature pedunculate oak (Quercus robur L.; stands Q1 and Q2), and humidity are important, and highly species specific, silver birch (Betula pendula Roth.; stands B1 and B2), environmental triggers for survival and distribution of Corsican pine (Pinus nigra ssp. laricio Maire; stand P1), macro-detritivores (Warburg, 1964; Haacker, 1968; Meyer or Austrian pine (P. nigra ssp. nigra Arnold; stand P2) & Eisenbeis, 1985; Dias et al., 2013). On the other hand, (Table 1). The stands all had an abrupt forest edge, i.e. the spatial distribution of detritivores is also strongly they lacked a gradual transition with the adjacent open influenced by soil acidity and exchangeable base cations land, and were always oriented towards the southwest, (Kime, 1992; Van Straalen & Verhoef, 1997), as well as perpendicular to the prevailing wind direction. All were food quality (C/N ratio) (Hassall et al., 2002; David & located in the periphery of forest complexes, within a Handa, 2010; Gerlach et al., 2014). These environmental fragmented landscape dominated by agriculture and inten- parameters vary strongly along edge-to-interior gradients, sive livestock breeding. Forest edges were bordered by and we would therefore expect species distribution pat- grass pasture, extensively managed meadow (B1, Q1, and terns to be highly influenced by distance from the forest P2), or by arable land (B2, Q2, and P1). A road with edge. Detailed empirical data that could be used to inves- roadside verges (~20 m in total) was present between the tigate these patterns is, however, to our knowledge very grassland or arable land and the forest edge of stands Q1 scarce. and Q2. The stands bordering arable land could have In northern Belgium, where forests are strongly frag- been exposed to drift of lime or fertiliser from agricultural mented, we investigated the distribution patterns of woo- applications. Under-storey vegetation was absent in all dlice and millipedes in transects that stretched from the stands except for (i) brambles (Rubus fruticosus agg.), forest edge towards the forest interior, for several differ- which occurred in the first 20 m from the edge in Q1 and ent forest stands (De Schrijver et al., 2007). On these further than 50 m from the edge in P1, and (ii) creeping acidic sandy forest soils, earthworms are very scarce or soft grass (Holcus mollis L.), which was present in the first even absent (Muys & Lust, 1992), and therefore woodlice 10 m from the edge in both P1 and P2. No significant and millipedes are the major macro-detritivore groups quantity of coarse woody debris was present in the stands. (Jeffery et al., 2010). Based on our knowledge of abiotic The edge patterns of soil nitrogen leaching, soil Ó 2016 The Royal Entomological Society, Insect Conservation and Diversity, 9, 456–469 458 Pallieter De Smedt et al. Table 1. Overview of the characteristics of the six investigated forest stands. Stand Location Dominant tree species Other species* Stand age (y) Soil pH(KCl) Q1 51°24044″N Quercus robur L. Alnus glutinosa L. 74 2.88 05°02045″E Sorbus aucuparia L. Q2 50°52008″N Quercus robur L. Alnus glutinosa L. 96 3.35 03°27059″E Prunus serotina Erhr. Sorbus aucuparia L B1 51°23030″N Betula pendula Roth Pinus nigra ssp. laricio Maire 35 3.40 05°02031″E Larix spec. Mill. B2 51°090220’N Betula pendula Roth Quercus robur L. 36-46 2.93 03°040480’E Sorbus aucuparia L. P1 51°26037″N Pinus nigra ssp. nigra Arnold Sorbus aucuparia L. 49 2.89 05°05014″E P2 51°08026″N Pinus nigra ssp. laricio Maire Betula pendula Roth 71 2.91 03°06036″E Quercus robur L.